The present invention relates to a laminated sheet metal used for container manufacture and a primer used in conjunction with same. More particularly, the present invention relates to a laminated sheet metal and a novel primer used to bind a thermoplastic resin film to the sheet metal resulting in enhanced processability and corrosion resistance.
Conventional methods used to produce cans result in two general types. A three piece can is known having a can body with a seam on the side. The seam is formed by soldering, adhesion, welding or similar processes. A top cover and a bottom cover are crimped on to the top and the bottom of the can body, respectively.
A conventional two-piece can has a seamless can body. The seamless can body is formed by drawing, deep-drawing with bending, drawing and ironing, impact forming (or stamping), and by similar methods. A container having a flange with a heat-sealed cover is also known. This known can is used as a light-weight container. A drawing process is used to form this conventional two piece can from coated metallic foil.
Metallic materials used for any of the cans above require protective coatings on both inside and outside surfaces. This is required to prevent corrosion and leaching of the metal. In order to reduce material costs and increase productivity, most metallic materials are now pre-coated prior to processing the metallic materials into cans.
Coated metallic materials used to make cans thus require superior endurance properties. These coated metallic materials have endurance properties that must be maintained in the face of severe processing conditions. Coated metallic materials also need superior sealing and corrosion resistance properties.
Generally, prior art methods of manufacturing cans utilize epoxy resins as coating materials. The use of epoxy resins is preferred due to their corrosion resistance and their ability to adhere to metallic materials. An epoxy resin may be used in combination with other curing resins, such as a phenol resin. The phenol resin is added to cure the epoxy resin.
Japanese Laid Open Patent Publications Nos. 59-15458, 1-284431 and 2-286709, disclose examples of prior art epoxy resins. These disclosures include a variety of epoxy resins modified by at least one of an aliphatic acid, a dibasic acid, a polyamidodicarboxylic acid and similar acids.
Conventional processes for metal sheet formation now include the step of laminating thermoplastic resin films to metallic materials with a primer, instead of coating thermosetting resins directly onto the metallic materials. Japanese Laid-open Patent Publication No. 62-10188 discloses such a process for use in the packaging industry.
Similarly, Japanese Laid Open Patent Publication No. 62-10188 discloses a process which includes the formation of a layered body. The layered body includes a polyester based film thermally adhesed to a metallic foil substrate. A thermosetting primer is sandwiched between the foil substrate and the film. The polyester based film is heat-sealable. The thermosetting primer further includes an epoxy resin component. The epoxy resin component includes about 450 to about 5500 units of an epoxy equivalent. The epoxy resin component further includes a curing resin component having one or more functional groups effective to react with the epoxy resin component to form a coating.
Japanese Laid Open Patent Publication No. 62-10188, further describes a thermosetting primer having a gel fraction ranging from about 50 to about 100 percent. The thermosetting primer is extracted in chloroform at 60.degree. C. for 60 minutes. In order to improve corrosion resistance properties of the layered body the polyester based film is disposed in a continuous, unbroken skin thereupon. The thermosetting primer formed from the epoxy-phenol resin exhibits improved adherence properties.
The layered body according to Japanese Laid-open Patent Publication No. 62-10188 endures severe processing conditions. This includes the retort sterilization step required for the canning process. However, the process according to Japanese Laid Open Patent Publication No. 62-10188 is plagued by numerous drawbacks. Chief among these are leaching of the involved metal and under-film corrosion. These difficulties occur when laminating a thermoplastic resin film on a metallic sheet containing a epoxy-phenol resin primer.
Forming cans by prior art processes (by stretching, deep-stretching, and deep-stretching with thin wall formation at side walls of the container) creates products having serious durability limitations.
Generally, products manufactured according to conventional processes, such as drawing or re-drawing yield inferior cans. A major drawback with the known processes is that cans are produced having an uneven height, and an improperly sized circumference (generally smaller than the desired dimension). It is thought that plastic flow of the sheet metal during the process is responsible for the uneven height and circumference of cans.
Further, cans manufactured according to conventional bending and stretching processes also have bodies with thinner walls than desired. Such thinner walls have low endurance properties and are easily deformed when subjected to stresses.
Finally, in a deep drawing process designed to form cans with thin walls, a conventional, epoxy-phenol resin-based primer tends to break and peel off. This is again due to the plastic flow of the sheet metal and the poor quality of adherence to resin films. Under-film corrosion and the leaching of metals are similarly caused by these drawbacks of the prior art.
Many similar problems exist during the manufacture of cans having conventionally known "easy open ends". Generally, scoring and riveting are employed during the manufacture of cans having such easy open ends. Scoring is used with dies and pre-coated can covers. Riveting is employed at the coated can cover to fasten tabs with rivets. The inside surface of the processed area tends to be damaged easily. This also results in more serious under-film corrosion and leaching problems, as discussed above.